The present invention relates to displays and more particularly to liquid crystal displays with minimized reflected ambient light.
As shown in FIG. 1, a prior art liquid crystal display (LCD) 9 typically comprises a means for changing the polarization of light, such as a twisted nematic liquid crystal cell (LCC) 10, with means 11 for illuminating the cell in the transmission mode with polarized light on one side 10b of the LCC 10. The illuminating means 11 typically comprises a source 12 of light that is partially collimated, i.e., in the plane of FIG. 1, and a linear polarizer 13. The source 12 comprises a lamp 12a, such as a line source, and a reflector 12b. The polarization direction of the polarizer 13 is at a 45.degree. angle with respect to the direction of the lamp 12a. Drive means 15 provides an electrical signal to the LCC 10 representing a desired image to be displayed. The midpoint director of the LCC 10 is perpendicular to the line source 12a direction. First means for analyzing the polarization of the transmitted polarized light 14 is positioned on the other side 10a of the LCC 10. The analyzing means 14 has its direction of polarization oriented perpendicular to the direction of polarization of the polarizer 13. A means for diffusing light 16, such as a diffuser screen, is placed between the analyzing means 14 and a viewer 18 to spread out the collimated light, thereby increasing the viewing angle.
In a normally lighted room, the contrast ratio of this arrangement is limited by reflected light 21 formed from ambient incident light 20 reflected off the curved surface 16a of the diffusing screen 16. This normally controls the total light from the display in the dark state, i.e., the screen 16, instead of being perfectly dark when the LCC 10 is in the dark state, provides some minimum light due to the reflected light 21. This is true, even though only about 5 percent (depending upon angle of incidence) of the incident light 20 is reflected from the curved surface 16a, if the ambient incident light 20 is strong enough. The reflected light from the flat surface 16b is not as much of a problem as is the reflected light from the curved surface 16a since the reflection from the surface 16b is typically specular and thus can be arranged to be out of the field of view of a viewer.
It is therefore desirable to have a display that minimizes reflected ambient light and thus provides a higher contrast ratio.